1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus including printers, copiers, and facsimiles, and an image density control method employed therein.
2. Description of the Background Art
In recent years, in addition to high imaging quality, durability and stability are expected of image forming apparatuses, such as printers, copiers, facsimile machines, and the like. In other words, it is necessary to minimize fluctuation in imaging quality and provide consistently stable imaging regardless of changes in operating environment or in operating conditions, such as continuous printing and intermittent printing.
Conventionally, a two-component developing method using a two-component developer composed essentially of a non-magnetic toner and a magnetic carrier (hereinafter referred simply as a developer) is widely known.
In the two-component developing method, the developer is borne on a developer bearing member (hereinafter referred to as a developing sleeve) including magnetic poles therein. The magnetic poles in the developer sleeve form a magnetic brush thereon. When a developing bias is supplied to the developing sleeve at a position facing a photoreceptor serving as a latent image carrier, a latent image on the photoreceptor surface is developed.
The two-component developing method is widely used because colorization is relatively easy with this method. In the two-component developing method, the developer is transported to a developing region by the rotation of the developing sleeve. When the developer is transported to the developing region, a number of magnetic carrier particles with toner particles in the developer are concentrated along magnetic lines of the magnetic poles, thereby forming the magnetic brush.
More than in a single-component developing method, in the two-component developing method it is important to control accurately a weight ratio of a toner and a carrier, that is, a toner concentration, in order to enhance stability. For example, when the toner concentration is too high, contamination of a background in an image and/or reduction in the resolution of fine images may occur.
By contrast, when the toner concentration is too low, the toner concentration of a solid portion of an image may decrease or the carrier may stick inadvertently. For this reason, it is necessary to regulate an amount of toner supply so that the concentration of toner in the developer is properly maintained.
One common method of regulating the toner concentration involves, for example, comparing an output value Vt of a toner concentration detector (such as a permeability sensor) to a toner concentration control reference value Vtref. In accordance with a difference between Vt and Vtref obtained, the amount of toner to be supplied is calculated, thereby enabling a toner supply device to supply the toner to a developing device in the proper amount.
The above-described method using the permeability sensor is one common method of detecting toner concentration. In this method, a change in the permeability of the developer caused by a change in the toner concentration represents a change in the toner concentration.
Another known method for detecting the toner concentration uses an optical sensor. In this method, a reference pattern is created on the image bearing member or an intermediate transfer belt and scanned with LED light. Reflected light (specular light or diffuse reflection) from the reference pattern is detected by an optical sensor such as a photodiode and a phototransistor. Based on a result provided by the optical sensor, the toner concentration or an amount of toner adhered to the reference pattern can be detected.
In a variation of the above-described approach, a reference pattern (a reference toner pattern) is created between recording sheets. In other words, the reference pattern is created at certain intervals (time or distance) between a previous imaging operation and a subsequent imaging operation. The photosensor detects reflected light from the reference toner pattern, thereby controlling the toner concentration control reference value Vtref.
Thus, for example, in a method for controlling image density disclosed in Japanese Patent Unexamined Application Publications Nos. Sho57-136667 and Hei02-34877, a toner pattern is formed in a non-image portion of an image and a detector detects a pattern density of the toner pattern. In accordance with the density of the toner pattern, a target value for the toner concentration control is adjusted to maintain image density.
However, a drawback of forming the toner pattern at the intervals between the previous and the subsequent transfer sheets is unnecessary toner consumption. Consequently, there is strong market demand for reducing the amount of toner consumed to produce the toner pattern. For this reason, when correction of the toner concentration is performed by forming the reference toner pattern between transfer sheets, either the frequency of formation of the toner patterns tends to be reduced or no reference toner pattern is formed at all.
Further, in a case in which the toner pattern is formed on the intermediate transfer belt and the secondary transfer roller is not separated from the intermediate transfer belt for each image-forming operation, a cleaning device is needed to remove the toner from the reference pattern that adheres to the secondary transfer roller.
By contrast, when the secondary transfer roller is separated from the intermediate transfer belt every time an image-forming operation is finished or after a certain number of image-forming operations, no cleaning device may be needed. However, in this case, mechanical durability of the structure is required in order to accommodate repeated separation of the secondary transfer roller from the intermediate transfer belt. In addition, when the secondary transfer roller separates from and contacts the intermediate transfer belt it generates vibrations that may show up as banding in an image.
As described above, it is desirable to reduce the frequency of formation of the toner patterns, for reasons of both imaging quality and cost reduction.
Accordingly, Japanese Patent No. 3410198 discloses ways in which the toner concentration may be reliably maintained. According to Japanese Patent No. 3410198, when the amount of toner supplied is controlled using the toner concentration sensor, fluctuation in the output of the toner concentration sensor caused by fluctuation in fluidity of the developer due to the duration of agitation is corrected.
However, even if a certain toner concentration is maintained, when developability of the developer is not stable, that is, when an amount of charge on the toner is not consistent, it is difficult to maintain the image density reliably solely by maintaining consistent toner concentration sensor output.
As a result, recently there have appeared image forming apparatuses that use methods for preventing the developing device from stressing the toner, such as adding additives such as silica (SiO2), titanium oxide (TiO2), or the like to the surface of the toner in order to enhance dispersion of the toner in image forming apparatuses using a two-component color developer.
However, such additives are susceptible to degradation due to mechanical stress or heat. Consequently, when being agitated in the developing device, such additives may be absorbed into the toner or separated inadvertently from the toner surface, causing fluctuation in fluidity and/or charging characteristics of the developer. Further, physical adhesion properties of the toner and the carrier may also change.
Moreover, when the stress generated by the developing device is reduced, the toner charging ability, that is, the ability of the developing device to charge the toner, may deteriorate for the following reason.
When an image having a relatively low ratio of an image area to the total area of the image is output, that is, when a relatively small amount of the toner is replaced per unit of time or per unit of sheets, the developability is maintained consistently. In other words, a slope of a graph, plotting amount of the toner developed against developing bias constant.
By contrast, when an image having a relatively large ratio of the image area to the total area of the image is output, that is, when a relatively large amount of the toner is replaced per unit of time or per unit of sheets, the developability may increase.
In other words, the developability changes depending on the amount of the toner replaced in the developer. This means that the developability changes even if the toner concentration does not change. Consequently, the toner concentration control reference value needs to be adjusted in order to maintain consistent developability over time.
However, there is a problem in that, when the image area ratio is relatively high, the toner concentration may not be maintained reliably simply by adjusting the toner concentration control reference value.
In view of the above, conventionally, electric potential is regulated during printing of an image having a high image area ratio so as to adjust image forming bias for forming the image and thus stabilize the image density.
According to this related-art approach, a print job is temporarily halted and the apparatus is put into an adjustment mode. Reference toner patterns of approximately 10 gradations are formed on the intermediate transfer belt, and the densities thereof are detected by the photosensor. According to a formula that relates the developing potential and the amount of the toner adhered, an appropriate developing bias can be obtained. Subsequently, the apparatus is returned to a print mode and printing is resumed.
However, with this configuration, the designated reference pattern for adjustment of the potential needs to be formed and detected, thereby generating more downtime for the image forming apparatus.